Bisulphite blocked NCO prepolymers in aqueous solutions and their use for shrink proofing materials which contain keratin are known in the art. These known substances have numerous important advantages. They are sufficiently water-soluble to be used in the form of aqueous solutions and yet they have the advantage of increasing the water repellence of keratin-containing material treated with them. Another advantage of bisulphite blocked NCO prepolymers is the solubility of the blocking agent in water and the consequent ease with which it may be washed-out of the treated material. In addition, compared with phenols which are otherwise preferably used as blocking agents in polyurethane chemistry, the bisulphites are substantially physiologically harmless substances.
A serious disadvantage of the aqueous solutions of bisulphite addition products used as shrink proofing agents such as those described in German OS No. 2,307,563 and South African Pat. No. 73,868, is their insufficient stability in storage, particularly at elevated temperatures, e.g. from 40.degree. to 50.degree.C. When ready for use, shrink proofing substances are often stored for many weeks or months in the open under summer or even subtropical conditions before they are used, in particular, for the treatment of wool. This consideration is particularly important for tropical countries but should not be underestimated for mid-European countries. The shrink proofing agents described in German OS No. 2,307,563 and South African Pat. No. 73,868 have the disadvantage of becoming unusable after only a few days at temperatures above 30.degree.C. This is due to the insufficient stability of the bisulphite adducts in the presence of water (phase separation and gelling occur).